/**
 * Licensed to DigitalPebble Ltd under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * DigitalPebble licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package com.digitalpebble.stormcrawler.protocol;

import java.time.ZoneId;
import java.time.ZoneOffset;
import java.time.ZonedDateTime;
import java.time.format.DateTimeFormatter;
import java.time.format.DateTimeParseException;

/**
 * A collection of HTTP header names and utilities around header values.
 * 
 * @see <a href="http://rfc-ref.org/RFC-TEXTS/2616/">Hypertext Transfer Protocol
 *      -- HTTP/1.1 (RFC 2616)</a>
 */
public interface HttpHeaders {

    public static final String TRANSFER_ENCODING = "transfer-encoding";

    public static final String CONTENT_ENCODING = "content-encoding";

    public static final String CONTENT_LANGUAGE = "content-language";

    public static final String CONTENT_LENGTH = "content-length";

    public static final String CONTENT_LOCATION = "content-location";

    public static final String CONTENT_DISPOSITION = "content-disposition";

    public static final String CONTENT_MD5 = "content-md5";

    public static final String CONTENT_TYPE = "content-type";

    public static final String LAST_MODIFIED = "last-modified";

    public static final String LOCATION = "location";

    /**
     * Formatter for dates in HTTP headers, used to fill the
     * &quot;If-Modified-Since&quot; request header field, e.g.
     * 
     * <pre>
     * Sun, 06 Nov 1994 08:49:37 GMT
     * </pre>
     * 
     * See <a href=
     * "https://www.w3.org/Protocols/rfc2616/rfc2616-sec3.html#sec3.3.1">sec.
     * 3.3.1 in RFC 2616</a> and <a
     * href="https://tools.ietf.org/html/rfc7231#section-7.1.1.1">sec. 7.1.1.1
     * in RFC 7231</a>. The latter specifies the format defined in RFC 1123 as
     * the &quot;preferred&quot; format.
     */
    public static final DateTimeFormatter HTTP_DATE_FORMATTER = DateTimeFormatter.RFC_1123_DATE_TIME
            .withZone(ZoneId.of(ZoneOffset.UTC.toString()));

    /**
     * Format an ISO date string as HTTP date used in HTTP headers, e.g.,
     * 
     * <pre>
     * 1994-11-06T08:49:37.000Z
     * </pre>
     * 
     * is formatted to
     * 
     * <pre>
     * Sun, 06 Nov 1994 08:49:37 GMT
     * </pre>
     * 
     * See {@link #HTTP_DATE_FORMATTER.}
     */
    public static String formatHttpDate(String isoDate) {
        try {
            ZonedDateTime date = DateTimeFormatter.ISO_INSTANT.parse(isoDate,
                    ZonedDateTime::from);
            return HTTP_DATE_FORMATTER.format(date);
        } catch (DateTimeParseException e) {
            // not an ISO date
            return "";
        }
    }
}
